Coal may be refined by a direct liquefaction process wherein the coal is liquefied by subjecting it to a hydrogen donor solvent in the presence of a hydrogen rich gas at elevated temperature and pressure. After dissolution the products are separated into gaseous material, distillate fractions and vacuum distillation bottoms. The residum containing entrained mineral matter and unconverted coal macerals is subjected to a solid/liquid separation, or deashing step, which can be any of several methods known to those skilled in the art. From the dashing step one or more streams of solvent refined coal (herein also referred to as "SRC") products are obtained which are free of ash minerals and unconverted coal. Desired SRC products include pentane soluble oils useful as liquid fuels, and solids, both of which are low in sulfur content.
The coal typically subjected to a direct liquefaction process is usually specified as being of a rank lower than anthracite, such as bituminous, sub-bituminous or lignite coals or mixtures thereof. Typically, the direct liquefaction process is not dependent on whether such coals are used directly from the mine, (e.g. "run-of-mine" coal) or whether they are pretreated (e.g. washed) to any of several levels to remove a portion of the entrained mineral matter. The coal, either run-of-mine or washed coal processed through a coal preparation plant, is ground to a size typically less than 8 mesh (Tyler Screen Classification), or more preferentially less than 20 mesh, and is dried to remove substantial moisture to a level for bituminous or sub-bituminous coals of less than about 4 percent by weight. The improved process of the invention employs a specific selection process which reflects upon the coal's composition and makes possible improved results upon subjection to direct liquefaction.
Coals are complicated mixtures of various distinct carbonaceous and non-carbonaceous materials found in nature. Due to the mechanism of geological formation of coals, they are nearly never found to be uniform in composition.
Not only are tremendous differences found in the coals taken from different seams within any particular area, but considerable differences are observed even within coals found in a particular seam. To those knowledgeable in coal composition, even coals within a narrow finite geographical area may differ considerably in composition, both as to type and amounts of mineral matter, as well as type and amounts of carbonaceous maceral composition.
Within any given mine the uniformity of the coal may vary to some degree, but during the mining process, the coal strata are mixed and intermingled. This tends to average out these greater distances along a particular coal strata, the differences may be so great between different mines or portions of the strata that even the intermingling and blending associated with removal of the coal often yields mined coals which differ significantly in their properties and composition.
Differences in coals are reflected in the quantity of minerals, their specific types and form of occurence, as found in nature. Between mines the relative amounts of iron minerals, chloride ion or calcium materials may differ significantly. The carbonaceous materials will also differ significantly between mines or even different portions of a large coal strata.
One method of improving the value of coal being removed from a seam is physical beneficiation, wherein "run-of-mine" material is separated by conventional techniques which take advantage of physical structure of the coal to remove mineral matter. Typically, one quarter to three quarters of the mineral matter is separated and removed, without significant loss in organic fuel value of the resulting "washed" coal.
Fortunately, pyrite, a sulfur-rich mineral the sulfur content of which is referred to herein as "pyritic sulfur", is one material that can be readily eliminated by treatment of run-of-mine coal to give lower sulfur-content products that burn in a more environmentally acceptable manner.
Several methods by which coal is treated to free it from undesirable inorganic elements are known by those skilled in the art and can be employed in accordance with the invention. Many of these techniques utilize gravity separation methods, since the inorganic material is more dense than the valuable carbonaceous components. For example, in the process of crushing coal, some of the mineral material is freed from the carbonaceous material. Generally, the smaller the crushed particles, the more impurities (i.e. minerals) are freed. As particles are generated, a sizing step may be employed to reject or recycle the larger particles. The crushed material can be subjected to a washing step, in which insoluble impurities are separated on the basis of their inherently greater specific gravity. In one such method, known as jigging, particles are stratified by water pulsation into a lighter fraction, which comprises mainly the carbonaceous components, and a heavier fraction which contains impurities. In another conventional coal washing process, a dense media is used which cleans by specific gravity. The heavier mineral materials do not float in the fluid slurry, whereas the carbonaceous materials do float and can be separated. As practiced in the industry, the dense media systems are commonly generated by suspending finely ground magnetite or sand in water to various levels having different specific gravities.
Other washing processes can also be utilized on finely ground coal particles. Dense-media cyclones, concentrating tables and floth flotation cells are familiar to those skilled in the art. All of the above methods serve to enrich the carbonaceous material by separating out refuse and mineral matter and can be utilized in accordance with the invention to provide a washed feed coal having substantial amounts of mineral matter removed. By removing as much mineral and refuse material as possible by the conventional methods of jigging, dense media separation or like means, the refuse that may be fed to the gasification unit in the liquefaction process can be minimized. Likewise, by removing the maximum amount of pyrite, the process demands for expensive hydrogen to convert the pyrite to hydrogen sulfide and pyrrhotite, which occurs under the operating conditions of direct liquefaction, is minimized. Keeping the hydrogen sulfide to a minimum likewise reduces the size of the gas scrubbing equipment.
In the liquefaction process, these washed or beneficiated coals have excellent potential, because much of the undesired mineral material is kept from entering the reaction system. Although many potential benefits of such coal preparation to the liquefaction process are known in the art, there are considerable differences in the way that various washed coals will behave in the liquefaction process. The nature of the carbonaceous fraction of coals is believed to be an important factor effecting the degree of coal conversion that will occur. For purposes of this invention, "coal conversion" means the relative amount of reacted (i.e. liquefied) coal to the total coal values processed.
It has long been recognized that liquefaction is heavily dependent on the maceral and, in particular, the vitrinite content of the feed coal. Fusinite, on the other hand, is the maceral most commonly associated with lack of conversion. Persons skilled in coal characterization art commonly group macerals into a group termed "total reactive macerals", which as used herein refers to the sum of the vitrinite, pseudovitrinite sporinite, resinite, cutinite, micrinite, and one third (1/3) of the semifusinite.
American coals that contain a large amount of total reactive macerals generally have been considered good candidates for the liquefaction process. However, experience has taught that even though coals may have similar contents of total reactive macerals, the degree of liquefaction and the relative product distributions still differ considerably. It has been recognized by Given et al in an article entitled "Dependence of Coal Liquefaction Behavior on Coal Characteristics 2. Role of Petrographic composition", which was published in FUEL, Vol. 54, January 1975, that petrographic composition is an important factor in determining liquefaction behavior. However, these authors indicate that the composition of the inorganic matter in the coal may be the most significant factor and that while maceral distribution is an important factor, the effects of various macerals was not well enough understood to serve as a basis for making confident predictions.
In U.S. Pat. No. 4,227,991 to Carr et al, coal conversion and yields of pentane soluble oils are enhanced by controlling the content and particle size of mineral solids having catalytic effect, including pyrite, which are of median diameter. While it is disclosed that a variety of feed coals can be used and, preferably those which upon dissolution generate smaller and more catalytically active inorganic mineral residue, the principle technique taught is to recycle process slurry containing the desired inorganic mineral matter and to "spike" this recycle stream with pyrite, as an additive. This increases the pyrite content of the slurry being subjected to liquefaction, but also results in increased levels of hydrogen consumption.
Thus, there exists a need, which is fulfilled by the present invention, for a reliable method by which to select coals for processing by direct liquefaction to obtain improved coal conversion and also to increase yields of higher fuel value pentane soluble coal-derived oils, preferably without high levels of mineral matter and hydrogen consumption. Such an ability to identify and selectively process coals that offer better levels of conversion and better product distributions offers the potential of carrying out a more economically and technically advantageous direct liquefaction process.